Clarke, P G H (1972) Visual motion-evoked potentials in man. Doctoral thesis, Keele University.

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Abstract

Visual motion-evoked potentials were recorded from the human scalp.
The stimulus chosen for most detailed study was sudden reversal of the motion of a patterned field, on the hypothesis that this was likely to activate only mechanisms selectively sensitive to the direction of stimulus motion. A large proportion of the experiments were designed to test this hypothesis; and in fact they supported it.
In addition to motion-reversal VEPs, VEPs to the onset and the offset of pattern motion, and to the appearance and disappearance of patterns were recorded and analysed. The relationships between these different types of VEP were investigated.
Also, the dependence of the motion-onset, -offset end -reversal VEPs on certain stimulus parameters was studied. Are motion-reversal VEPs produced by direction-selective mechanisms?
That direction-selective mechanisms were at least partly responsible for the motion-reversal VEPs was confirmed, since an adapting stimulus moving in the same direction as the motion before reversal produced an effect on the VEP different to that produced by en adapting stimulus moving in the opposite direction.
Further investigation indicated that direction-selective mechanisms were probably the sole contributors to the motion-reversal VEPs, since control experiments failed to support any of the most likely alternative ways in which direction-insensitive mechanisms might theoretically have contributed to the motion-reversal VEPs.
In particular, considerable attention was devoted to the possibility that mechanisms sensitive to contrast but insensitive to direction of motion might have been activated by a brief increase in the effective contrast of the stimulus pattern at the moment of reversal, and thereby have contributed to the VEP. Such an increase in the effective contrast could in theory have been caused by the brief slowing down which inevitably occurred at the moment of reversal, but several experiments refuted this interpretation. In particular, the VEPs were virtually independent of the time taken for reversal, but were very dependent on the velocity before and after reversal, reducing almost to zero at very high or very low velocities.
A sudden step-displacement or change of the pattern at the moment of reversal suppressed the VEP. This effect was not caused by interference with the time-course of slow movement at reversal, since suppression occurred even when the step-displacement took place outside the period of slow movement. A psychophysical effect has been observed which may be connected with this phenomenon.
Involuntary eye movements ere apparent~ not implicated in the production of the VEPs, since periodic and aperiodic stimulation yield similar results.
Certain other ways in which VEP components might have arisen, even in the absence of eye movements or imperfections in the stimulus motion, have been investigated; but there has been no indication of the occurrence of such components.
So the motion-reversal VEPs probably arose almost entirely from direction-selective mechanisms.

Component analysis of VEPs
The VEPs to the reversal and to the offset of motion apparently comprised three separate component peaks. In this respect they were similar to pattern-appearance VEPs, and the distribution over the scalp of any one of the components was the same for all three kinds of VEP (e.g. the first peak of the motion-reversal VEP had the some scalp-distribution as the first peak of the motion-offset VEP and the first peak of the pattern-appearance VEP). This implied that the corresponding components originated in the same cortical areas, and a correlation analysis of the amplitudes of the various components of motion-reversal VEPs and pattern-appearance VEPs for different subjects supported this conclusion. Now there is convincing evidence (Jeffreys, 1971) that the first component of pattern-appearance VEPs originates in striate cortex end the later components in extrastriate cortex. It is therefore concluded that the first peaks of motion-reversal and. motion-offset VEPs are likewise probably from striate cortex, and the later peaks from extrastriate.
The VEP to motion-onset was very different from the above VEPs, however, and appeared to be more closely related to the pattern-disappearance VEP. It is possible that the same mechanisms underlie these two kinds of VEP.
Although motion-reversal VEPs appear to be the product of direction-selective mechanisms alone, it is far from certain that this is true of motion-onset and -offset VEPs. Nevertheless, there is evidence that the latter kinds of VEP may share generating mechanisms with the former; since the motion-reversal VEP was, under many conditions though not all, a good approximation to the sum of the motion-onset and -offset VEPs recorded under similar stimulus conditions.
The effects of varying stimulus parameters
Motion-reversal VEPs were found to be largely independent of brightness except at the lowest levels, but the latency did tend to increase slightly as the brightness was reduced.
Despite the discovery (MaCKay & Rietveld, 1968) that the proximity of a stationary reference line enhances the VEP to the onset of motion of a stimulus line,' it appears that the sharp contours comprising the edge of the visual field did not influence the VEPs to the onset, offset or reversal of pattern motion, since replacing the sharp contours by blurred ones did not affect the VEPs.
The onset, offset and reversal VEPs did not depend greatly on the direction of motion. Superimposing a steady motion did, however, markedly modify the VEPs.
The effects of using patterns other than visual noise were investigated. Checkerboards and visual noise produced similar results, but line rasters produced very different VEPs.

Item Type: Thesis (Doctoral)
Subjects: R Medicine > RC Internal medicine > RC0321 Neuroscience. Biological psychiatry. Neuropsychiatry
Divisions: Faculty of Natural Sciences > School of Life Sciences
Depositing User: Lisa Bailey
Date Deposited: 27 Feb 2019 12:18
Last Modified: 27 Feb 2019 12:18
URI: http://eprints.keele.ac.uk/id/eprint/5927

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